Can they be admitted back once they change their mind?
I've to admit MTF hasn't been the same ever since his departure.

Sadly, no.
A case arose while I was Appeals Mod, (ServeVolley), and the appeal was unsuccesful. The argument was that the poster is reminded that his/her decision is irrevocable before the ban takes effect, so if he/she goes ahead with the request, that's it.